AN2950
Application note
EVLVIPER28L-10W: 5 V/10 W, 60 kHz isolated flyback
with extra power management
Introduction
This document describes a 5 V, 2 A application with 3.3 A peak current capability of 1.9 sec,
using VIPer28, a new offline high-voltage converter from STMicroelectronics.
In some applications, an SMPS sometimes undergoes load peaks that can be two or more
times as much as the power it is supposed to deliver, though only for a short time interval
compared to the thermal time constants of the power components. Typical examples of such
loads are printers and audio systems.
In such cases, it is more cost-effective to thermally design the system for the maximum
continuous power and not for the peak power demand, which is sustained only for a limited
time window.
Such a design is possible thanks to the EPT function of the VIPer28, which allows the
designer to fix the maximum time window during which the converter is able to manage the
peak power and still maintain output voltage regulation. If the overload lasts for longer than
this time window, the converter is automatically shut down and enters auto-restart mode
until the overload is removed, so as to prevent damage to the power components.
The device has many other features such as an 800 V avalanche rugged power section,
PWM operation at 60 kHz with frequency jittering for lower EMI, a limiting current with
adjustable setpoint, an on-board soft-start, a safe auto-restart after a fault condition, and a
low standby power (< 50 mW at 265 VAC).
The available protections include adjustable and accurate overvoltage protection, thermal
shutdown with hysteresis and delayed overload protection.
Figure 1.
April 2010
Demonstration board
Doc ID 15494 Rev 1
1/37
www.st.com
Contents
AN2950
Contents
1
2
3
Adapter features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1
Circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2
Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Testing the board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1
Typical board waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2
Regulation precision and output voltage ripple . . . . . . . . . . . . . . . . . . . . 12
2.3
Burst mode and output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5
Light load performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.6
Overload protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.7
Second OCP protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.8
Output overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.9
EPT function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.10
Thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.11
EMI measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.12
Board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Appendix A Test equipment and measurement of efficiency and low load
performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
A.1
Notes on input power measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2/37
Doc ID 15494 Rev 1
AN2950
List of tables
List of tables
Table 1.
Table 2.
Table 3.
Table 4.
Table 5.
Table 6.
Table 7.
Table 8.
Table 9.
Table 10.
Table 11.
Table 12.
Table 13.
Table 14.
Table 15.
Table 16.
Table 17.
Table 18.
Table 19.
Table 20.
Electrical specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1338.0019 transformer characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Transformer pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Output voltage and VDD line-load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Burst mode related output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Active mode efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Input voltage averaged efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Energy efficiency criteria for standard models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Energy efficiency criteria for low-voltage models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
No load input power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Energy consumption criteria for no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Low load performance - POUT = 25 mW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Low load performance - POUT = 50 mW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Output power when the input power is 1 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Output overvoltage threshold at different input/output conditions. . . . . . . . . . . . . . . . . . . . 25
Temperature of key components at 115 VAC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Doc ID 15494 Rev 1
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List of figures
AN2950
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
Figure 39.
Figure 40.
Figure 41.
Figure 42.
Figure 43.
Figure 44.
Figure 45.
Figure 46.
Figure 47.
Figure 48.
4/37
Demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Transformer size and pin diagram - bottom view (pin side) . . . . . . . . . . . . . . . . . . . . . . . . . 9
Transformer size and pin diagram - electrical diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Transformer size - side view 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Transformer size - side view 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Drain current and voltage at maximum load 115 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Drain current and voltage at maximum load 230 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Drain current and voltage at maximum load 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Drain current and voltage at maximum load 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Drain current and voltage at peak load 90 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Drain current and voltage at peak load 265 VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Output voltage ripple 115 VIN_AC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output voltage ripple 230 VIN_AC full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output voltage ripple at 115 VIN_AC no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output voltage ripple at 230 VIN_AC no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Output voltage ripple at 115 VIN_AC 50 mA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Output voltage ripple at 230VIN_AC 50 mA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Efficiency vs VIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Active mode efficiency vs VIN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Input voltage averaged efficiency vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
ENERGY STAR® efficiency criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Pin vs VIN at low load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Pout vs VIN at Pout = 1 W . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
OLP: output short and protection tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
OLP: output short and protection tripping (zoom on FB pin voltage) . . . . . . . . . . . . . . . . . 21
OLP: steady state (autorestart mode). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
OLP: steady state (zoom on FB pin voltage) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
OLP: steady-state, short removal and restart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
OLP: restart after short removal (zoom) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Second OCP protection tripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Operating with secondary winding shorted. Restart mode . . . . . . . . . . . . . . . . . . . . . . . . . 22
Operating with secondary winding shorted. Steady state . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Operating with secondary winding shorted. Steady state (zoom). . . . . . . . . . . . . . . . . . . . 23
OVP circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Output overvoltage protection at 115 VIN_AC 0.2 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Output voltage ripple at 230 VIN_AC no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Auto-restart mode of the overvoltage protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Auto-restart mode of the overvoltage protection (zoom) . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Normalized output OVP threshold vs load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Extra power applied and removed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Extra power applied indefinitely . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
EPT thresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Thermal measurements at 90 VAC full load, Tamb = 25 °C . . . . . . . . . . . . . . . . . . . . . . . 27
Average measurement of background noise (board disconnected from mains). . . . . . . . . 28
Average measurement at 115 Vac, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Average measurements at 230 Vac, full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Doc ID 15494 Rev 1
AN2950
Figure 49.
Figure 50.
Figure 51.
Figure 52.
Figure 53.
List of figures
Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Connection of the UUT to the wattmeter for power measurements . . . . . . . . . . . . . . . . . . 32
Suggested connection for low power measurements (switch in position 1) . . . . . . . . . . . . 33
Suggested connection for high power measurements (switch in position 2) . . . . . . . . . . . 33
Doc ID 15494 Rev 1
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Adapter features
1
AN2950
Adapter features
Table 1 lists the electrical specifications of the demonstration board.
Table 1.
Electrical specification
Parameter
1.1
Symbol
Value
Input voltage range
VIN
[90 VRMS; 265 VRMS]
Output voltage
VOUT
5V
Maximum output current
IOUTmax
2A
Peak output current
IOUTpk
3A
Precision of output regulation
ΔVOUT_LF
±5%
High-frequency output voltage
ripple
ΔVOUT_HF
50 mV
Maximum ambient operating
temperature
TA
60°C
Circuit description
The power supply is set-up in a flyback topology. Its schematic is shown in Figure 2. The
input section includes the protection elements (fuse and NTC for inrush current limiting), a
filter for EMC suppression (C1, T2, C13), a diode bridge (BR1) and an electrolytic bulk
capacitor (C3) as the front-end AC-DC converter. The transformer uses a standard E25
ferrite core. A transil clamp network is used to demagnetize the leakage inductance.
At power-up, the DRAIN pin supplies the internal HV start-up current generator that charges
the C4 capacitor up to VDDon. At this point, the power MOSFET starts switching, the
generator is turned off and the IC is powered by the energy stored in C4 until the auxiliary
winding voltage becomes high enough to sustain the operation through D1 and R1.
The value of the resistor R3 between the CONT and GND pins is high enough that the
VIPer28’s current limit does not change with respect to the datasheet’s default value IDlim.
This resistor, in conjunction with D2, R14 and R15, is used to realize the overvoltage
protection and the feedforward correction function, as described further in this document.
The output rectifier D4 has been selected according to the calculated maximum reverse
voltage, forward voltage drop and power dissipation, and is a power Schottky type.
The output voltage regulation is performed by a secondary feedback with a TS431 driving
an optocoupler (in this case a PC817) ensuring the required insulation between the primary
and secondary. The opto-transistor drives directly the FB pin of the VIPer28, which is
connected to the compensation network made up by C6, C7 and R12.
A small LC filter has been added at the output in order to filter the high-frequency ripple
without increasing the size of the output capacitors, and a 100 nF capacitor has been placed
very close to the solder points of the output connector to limit the spike amplitude.
6/37
Doc ID 15494 Rev 1
Doc ID 15494 Rev 1
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Figure 2.
%5
AN2950
Adapter features
Application schematic
!-V
7/37
Adapter features
Table 2.
AN2950
Bill of materials
Reference
Part
Description
Manufacturer
BR1
DF06M
600 V 1 A diodes bridge
VISHAY
C1, C13
100 nF X2 capacitor
EVOX RIFA
C3
47 µF 450 V electrolytic capacitor
PANASONIC
C4
22 µF 35 V electrolytic capacitor
PANASONIC
C5, C9
Not mounted
C6
3.3 nF 25 V ceramic capacitor
EPCOS
C7
33 nF 50 V ceramic capacitor
EPCOS
C8
2.2 nF Y1 capacitor
CERAMITE
1000 µF 10 V ultra-low ESR electrolytic
capacitor
RUBYCON
C10
100 µF 10 V electrolytic capacitor
PANASONIC
C11
4.7 nF 100 V ceramic capacitor
EPCOS
C12
2.2 µF 50 V electrolytic capacitor
PANASONIC
C16
2.2 nF 100 V ceramic capacitor
AVX
C14, C15
MCZ series
D1
BAT46
Diode
STMicroelectronics
D2
1N4148
Small-signal, high-speed diode
NXP
D3
STTH1L06
Ultra-fast, high-voltage diode
STMicroelectronics
D4
STPS5L40
5 A - 40 V power Schottky rectifier
STMicroelectronics
D5
1.5KE300
Transil
STMicroelectronics
Dz
BZX79-C18
18 V Zener diode
NXP
F1
TR5 250 V 500 mA
Fuse
SCHURTER
L1
RFB0807-2R2L
2.2 µH
COILCRAFT
2.2 Ω thermistor
EPCOS
Opto coupler
SHARP
NTC1
OPTO1
PC817D
R1
4.7 Ω 1/4 W axial resistor
R3
68 kΩ 1/4 W axial resistor
R6
15 kΩ 1/4 W axial resistor
R8
82 kΩ 1/4 W axial resistor 1% tolerance
R9
27 kΩ 1/4 W axial resistor 1% tolerance
R10
560 kΩ 1/4 W axial resistor
R12
10 kΩ 1/4 W axial resistor
R13
3.3 kΩ 1/4 W axial resistor
R14
330 kΩ 1/4 W axial resistor
R15
2 MΩ 1/4 W axial resistor
Rz
68 Ω 1/4 W axial resistor
8/37
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AN2950
Table 2.
Adapter features
Bill of materials (continued)
Reference
Part
Description
Manufacturer
T2
BU16-4530R5BL
Common mode choke
COILCRAFT
VR1
TS431
Voltage reference
STMicroelectronics
U1
VIPER28LN
Offline high voltage controller
STMicroelectronics
T1
1338.0019
Switch mode power transformer
MAGNETICA
1.2
Transformer
Table 3 lists the electrical characteristics of the transformer.
Table 3.
1338.0019 transformer characteristics
Properties
Value
Test Condition
Primary inductance
1.5 mH ±15%
Measured at 1 kHz
Leakage inductance
0.8% nom.
Measured at 10 kHz
Primary to secondary turn ratio
(4 - 5)/(6,7 – 10,9)
12.85 ±5%
Measured at 10 kHz
Primary to auxiliary turn ratio
(4 - 5)/(1 - 2)
5.29 ±5%
Measured at 10 kHz
Nominal operating frequency
60 kHz
Nominal/peak power
10 W/15 W
Saturation current
1A
BSAT = 0.32T
Insulation
4 kV
Primary to secondary
The size, pinout and mechanical characteristics are given in the following figures.
Figure 3.
Transformer size and pin diagram - Figure 4.
bottom view (pin side)
Transformer size and pin diagram electrical diagram
!-V
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9/37
Adapter features
Figure 5.
AN2950
Transformer size - side view 1
Figure 6.
Transformer size - side view 2
!-V
Table 4.
!-V
Transformer pin description
Pin
Description
Pin
5
Primary, to the DC input voltage (400 V)
6
4
Primary, to the drain of the MOSFET
7
3
Removed
8
2
Auxiliary GND
9
1
Auxiliary output
10
Description
Secondary output
n.c
Secondary GND
10/37
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AN2950
Testing the board
2
Testing the board
2.1
Typical board waveforms
The drain voltage and current waveforms are reported for the two nominal input voltages
and for the minimum and maximum voltages of the converter’s input operating range.
Figure 7 and Figure 8 show the drain current voltage waveforms at nominal input voltages
(115 VAC and 230 VAC) and maximum load (2 A). Figure 9 and Figure 10 show the same
waveforms for the same load conditions, but with minimum (90 VAC) and maximum
(265 VAC) input voltages.
Figure 11 and Figure 12 show the same waveforms during peak load conditions (3.3 A),
which the converter is able to sustain for approximately 1.9 seconds while still keeping the
output voltage under regulation.
The converter is operated in continuous conduction mode (CCM) at low input voltage during
full load conditions and even at high input voltage during peak load conditions. CCM allows
reducing the value of the root mean square currents, both on the primary side (in the power
switch) and on the secondary side (in the output diode D4 and in the output capacitors C9
and C14), thus reducing the power dissipation and the stress on the power components.
Figure 7.
Drain current and voltage at
maximum load 115 VAC
Figure 8.
Drain current and voltage at
maximum load 230 VAC
!-V
Figure 9.
Drain current and voltage at
maximum load 90 VAC
!-V
Figure 10. Drain current and voltage at
maximum load 265 VAC
!-V
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Figure 11. Drain current and voltage at peak
load 90 VAC
Figure 12. Drain current and voltage at peak
load 265 VAC
!-V
2.2
!-V
Regulation precision and output voltage ripple
The output voltage of the board has been measured with different line and load conditions.
The results are reported in Table 5. The output voltage is not affected by the line condition.
The VDD voltage has also been measured to verify that it is within the device’s operating
range.
As confirmed by the results reported in Table 5, the VDD voltage (unregulated auxiliary
output) increases as the load on the regulated output increases. To prevent the VDD voltage
from exceeding its operating range, an external clamp has been used (Dz, Rz).
Table 5.
VIN_AC
(V)
Output voltage and VDD line-load regulation
No load
Half load
Full load
Peak load
VOUT (V)
VDD (V)
VOUT (V)
VDD (V)
VOUT (V)
VDD (V)
VOUT (V)
VDD (V)
90
5.02
10.8
4.99
18.8
4.99
19.6
4.98
20.5
115
5.02
10.4
4.99
18.7
4.99
19.5
4.98
20.3
230
5.02
10.3
4.99
18.6
4.99
19.4
4.98
20
265
5.02
10.1
4.99
18.5
4.99
19.2
4.98
19.8
The ripple at the switching frequency superimposed at the output voltage has also been
measured and the results are reported in Table 6. The board is provided with an LC filter to
better filter the voltage ripple.
Table 6.
Output voltage ripple
Half load
Full load
Peak load
VOUT (mV)
VOUT (mV)
VOUT (mV)
90
15
17
30
115
14
21
28
230
15
22
25
265
14
20
24
VIN_AC (V)
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Figure 13. Output voltage ripple 115 VIN_AC full Figure 14. Output voltage ripple 230 VIN_AC full
load
load
!-V
2.3
!-V
Burst mode and output voltage ripple
When the load is so low that the voltage at the FB pin falls below the internal threshold
VFBbm (0.6 V typical), the VIPer28 is disabled. At this point, the feedback’s reaction to the
stop of energy delivery makes the FB voltage increase again, and when it goes 100 mV
above the VFBbm threshold the device restarts switching. This results in a controlled on/off
operation referred to as "burst mode" and is shown in the figures below, which report the
output voltage ripple and the feedback voltage when the converter is not or only lightly
loaded and supplied with 115 VAC and 230 VAC respectively. This mode of operation keeps
the frequency-related losses low when the load is very light or disconnected and makes it
easier to comply with energy-saving regulations.
Figure 15. Output voltage ripple at 115 VIN_AC Figure 16. Output voltage ripple at 230 VIN_AC
no load
no load
!-V
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Figure 17. Output voltage ripple at 115 VIN_AC Figure 18. Output voltage ripple at 230VIN_AC
50 mA
50 mA
!-V
!-V
Table 7 shows the measured value of the burst mode frequency ripple measured in different
operating conditions. The ripple in burst mode operation is very low and always below
25 mV.
Table 7.
2.4
Burst mode related output voltage ripple
VIN
No load (mV)
25 mA load (mV)
50 mA load (mV)
90
5
14
15.4
115
5.6
18.2
20.2
230
6
19.2
22
265
6.5
20.2
24
Efficiency
This and the following section report the results of efficiency and light load measurements.
Appendix A at the end of this document provides some details on the settings of the power
measurement equipment.
According to the ENERGY STAR® average active mode efficiency testing method, the
efficiency measurements have been done at full load and at 75%, 50% and 25% of full load
for different input voltages. The results are reported in Table 8.
Table 8.
Efficiency
Efficiency (%)
VIN_AC (VRMS)
14/37
Full load
75% load
50% load
25% load
(2 A)
(1.5 A)
(1 A)
(0.5 A)
90
79.06
79.93
79.41
83.72
115
80.61
81.12
83.45
83.78
150
81.53
81.85
81.73
85.03
180
81.90
81.58
80.46
80.38
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Testing the board
Table 8.
Efficiency (continued)
Efficiency (%)
VIN_AC (VRMS)
Full load
75% load
50% load
25% load
(2 A)
(1.5 A)
(1 A)
(0.5 A)
230
82.07
82.13
80.06
80.39
265
81.96
81.94
81.16
80.36
For better visibility, the results have also been plotted in the diagrams below. Figure 19 plots
the efficiency versus VIN for the four different load values. Figure 20 reports the efficiency as
a function of the load for different input voltage values.
Figure 19. Efficiency vs VIN
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Figure 20. Efficiency vs load
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6AC
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The active mode efficiency is defined as the average of the efficiencies measured at 25%,
50% and 75% of the maximum load and at the maximum load itself. Table 9 reports the
active mode efficiency calculated from the values in Table 8. For clarity, the values in Table 9
are plotted in Figure 21.
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Testing the board
Table 9.
AN2950
Active mode efficiency
Active mode efficiency
VIN_AC (VRMS)
Efficiency (%)
90
80.53
115
82.24
150
82.53
180
81.08
230
81.16
265
81.36
Figure 21. Active mode efficiency vs VIN
!CTIVEMODEEFFICIENCYVS6IN
EFF;=
6IN;6AC=
!-V
In Table 10 and Figure 22 the averaged values of the efficiency versus load are reported
(the average has been done considering the efficiency at different values of the input
voltage).
Table 10.
16/37
Input voltage averaged efficiency
Load (% of full load)
Efficiency (%)
100
80.93
75
81.28
50
81.02
25
82.06
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Testing the board
Figure 22. Input voltage averaged efficiency vs load
)NPUTVOLTAGEAVERAGEDEFFICIENCYVSLOAD
EFF;=
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!-V
®
In version 2.0 of the ENERGY STAR program requirements for single voltage external
AC-DC power supplies(a), the power supplies are divided into two categories: low-voltage
power supplies and standard power supplies with respect to the nameplate output voltage
and current. To be considered a low-voltage power supply, an external power supply must
have a nameplate output voltage of less than 6 V and a nameplate output current of more
than or equal to 550 mA.
The following tables report the EPA energy efficiency criteria for AC-DC power supplies in
active mode for standard models and low voltage models respectively.
Table 11.
Energy efficiency criteria for standard models
Nameplate output power (Pno)
Minimum average efficiency in active mode
(expressed as a decimal)
0 to = 1 W
= 0.48 × Pno + 0.140
> 1 to = 49 W
= [0.0626 × In (Pno)] + 0.622
> 49 W
= 0.870
Table 12.
Energy efficiency criteria for low-voltage models
Nameplate output power
Minimum average efficiency in active mode
(Pno)
(expressed as a decimal)
0 to = 1 W
= 0.497 × Pno + 0.067
> 1 to = 49 W
= [0.075 × In (Pno)] + 0.561
> 49 W
= 0.860
The criteria are plotted in Figure 23, where the red line depicts the criteria for the standard
model and the blue line the criteria for the low voltage model. The PNO axis uses a
logarithmic scale.
a. Refer to Chapter : References.
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Figure 23. ENERGY STAR® efficiency criteria
!-V
The presented power supply belongs to the low-voltage power supply category and, in order
to be compliant with ENERGY STAR® requirements, needs to have an efficiency higher
than 73.37%. For all the considered input voltages, the efficiency (see Table 8) results are
higher than the recommended value.
2.5
Light load performances
The input power of the converter has been measured in no load conditions for different input
voltages. The results are reported in Table 13.
Table 13.
No load input power
VIN_AC (VRMS)
Pin (mW)
90
13.8
115
14.5
150
16
180
17
230
20
265
24
Version 2.0 of the ENERGY STAR® program also takes into consideration the power
consumption of the power supply when it is not loaded. The criteria to be compliant with are
reported in Table 14.
Table 14.
Energy consumption criteria for no load
Nameplate output power (Pno)
Maximum power in no load for AC-DC EPS
0 to = 50 W
< 0.3 W
> 50 W < 250 W
< 0.5 W
The performance of the presented board is much better than the requirement; the power
consumption is about twelve times lower than the ENERGY STAR® limit.
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Testing the board
Even if the performance seems to be unproportionally better than the requirement, it is
worth noting that often AC-DC adapter or battery charger manufacturers have very strict
requirements about no load consumption, and if the converter is used as an auxiliary power
supply, the line filter is often the big line filter of the entire power supply that significantly
increases consumption in standby mode.
Even if the ENERGY STAR® program does not have other requirements regarding light load
performance, in order to provide complete information we have also reported the input
power and efficiency of the demonstration board in two other low load cases. Table 15 and
Table 16 show the board’s performances when the output load is 25 mW and 50 mW
respectively.
Table 15.
Low load performance - POUT = 25 mW
VIN_AC
POUT (mW)
PIN (mW)
Eff. (%)
PIN-POUT (mW)
90
25
45.5
54.94505
20.5
115
25
46.8
53.4188
21.8
150
25
47.7
52.4109
22.7
180
25
50.4
49.60317
25.4
230
25
55
45.45455
30
265
25
61
40.98361
36
Table 16.
Low load performance - POUT = 50 mW
VIN_AC
POUT (mW)
PIN (mW)
Eff. (%)
PIN-POUT (mW)
90
50
73.5
68.02721
23.5
115
50
76.6
65.27415
26.6
150
50
80
62.5
30
180
50
82
60.97561
32
230
50
84
59.52381
34
265
50
92
54.34783
42
Figure 24 reports the input power vs input voltage for no load and low load conditions.
Figure 24. Pin vs VIN at low load
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Depending on the equipment supplied, there are several criteria to measure the standby or
light load performance of a converter. One of these is the measurement of the output power
when the input power is equal to one watt. Table 17 reports the output power needed to
obtain 1 W of input power in different line conditions. Figure 25 shows the results of this
measurement.
Table 17.
Output power when the input power is 1 W
VIN_AC
PIN (W)
POUT (W)
Eff. (%)
PIN-POUT (W)
90
1
0.81663
81.663
0.18337
115
1
0.83166
83.166
0.16834
150
1
0.81162
81.162
0.18838
180
1
0.78657
78.657
0.21343
230
1
0.77655
77.655
0.22345
265
1
0.77154
77.154
0.22846
Figure 25. Pout vs VIN at Pout = 1 W
EFF;=
2.6
6IN;6=
!-V
Overload protection
The VIPer28 is a current mode converter. This means that the regulation of the output
voltage is made by increasing or decreasing the primary peak current on a cycle-by-cycle
basis as a consequence of the increase or decrease of the output power demand.
The peak current is internally sensed and converted into a voltage that is compared with the
FB pin voltage. The device is shut down as soon as the two voltages become equal.
When the FB pin voltage reaches VFBlin (3.5 V typical), the drain peak current reaches its
maximum value, IDlim (which is 0.8 A typical or a lower value according to the value of the
resistor if connected between the LIM and GND pins).
If the load power demand exceeds the converter’s power capability, the FB pin voltage
exceeds VFBlin and the device waits for a certain time, fixed by the value of the capacitor C7,
before shutting down the system: the internal pull-up is disconnected and the pin starts
sourcing a 3 µA current that charges the capacitor.
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As the voltage on the feedback pin reaches the VFBolp threshold (4.8 V typical), the VIPer28
stops switching (see Figure 27) and is not allowed to switch again until the VDD voltage has
fallen below VDD(RESTART) (4.5 V typical) and risen again up to VDDon (14 V typical, see
Figure 26 and Figure 30).
If the short-circuit is not removed, the protection is tripped again and the system works in
auto-restart mode (see Figure 26, 28 and 30).
If the overload disappears, the converter resumes working normally at the first VDD
recycling, as shown in Figure 30 and 31.
C7 is needed because usually the value of the C6 capacitor, coming from the loop stability
calculations, is too small to ensure an OLP delay time long enough to bypass the initial
output voltage transient at start-up. The value of C7 can be sufficiently high to provide the
needed delay, and the value of R12 chosen so that the R12-C7 pole does not affect the
stability of the loop.
During an overload the converter is operated at a very low duty cycle, being the MOSFET
kept in the off state for most of the time. This results in a very low average power throughput,
which is safe for the power elements in this condition.
Figure 26. OLP: output short and protection
tripping
Figure 27. OLP: output short and protection
tripping (zoom on FB pin voltage)
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Figure 28. OLP: steady state (autorestart
mode)
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!-V
Figure 29. OLP: steady state (zoom on FB pin
voltage)
!-V
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Figure 30. OLP: steady-state, short removal
and restart
Figure 31. OLP: restart after short removal
(zoom)
6KRUWUHPRYHGKHUH
!-V
2.7
!-V
Second OCP protection
The VIPer28 is provided with a first adjustable level of primary overcurrent limitation that
switches off the power MOSFET if this level is exceeded. This limitation acts cycle by cycle,
and its main purpose is to limit the maximum deliverable output power. A second level of
primary overcurrent protection is also present, which is not adjustable but fixed to 1.2 A
(typical value). If the drain peak current exceeds this second overcurrent protection
threshold, the device enters a warning state. If, during the next on time of the power
MOSFET, the second level of overcurrent protection is exceeded again, the device assumes
that a secondary winding short-circuit or a hard saturation of the transformer is occurring
and stops the PWM activity. To re-enable the operation, the VDD pin voltage has to be
recycled, which means that VDD has to go down to VDD(RESTART), then rise up to VDDon. At
this point the MOSFET restarts switching. If the cause of activation of the second
overcurrent protection is still present, the protection is tripped again and the system works in
auto-restart mode, resuming normal operation as soon as the cause of the fault is removed
and the VDD recycled.
This protection has been tested in different operating conditions, short-circuiting the output
diode. The following figures show the behavior of the system during these tests.
Figure 32. Second OCP protection tripping
Figure 33. Operating with secondary winding
shorted. Restart mode
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Figure 34. Operating with secondary winding
shorted. Steady state
Figure 35. Operating with secondary winding
shorted. Steady state (zoom)
!-V
!-V
With faults like the ones described, the second OCP protection of the VIPer28 stops the
operation after two switching cycles, thus avoiding high currents in both the primary and
secondary windings and through the power section of the VIPer28 itself. Figure 33 shows
the operation when a permanent short-circuit is applied on the secondary winding. Most of
the time the power section of the VIPer28 is off, eliminating any risk of overheating.
Output overvoltage protection
During the power MOSFET’s off time, the voltage generated by the auxiliary winding tracks
the converter's output voltage through the transformer’s auxiliary-to-secondary turn ratio.
The diode D2 is forward biased, and the voltage divider made up by R14 and R3 (see
Figure 2 and Figure 36) between the auxiliary winding and the CONT pin performs an
output voltage monitor function: if the voltage applied to the CONT pin exceeds the internal
VOVP threshold (3 V typical) for four consecutive switching cycles, the controller recognizes
an overvoltage condition and shuts down the converter. This is done to provide high noise
immunity and avoid that spikes erroneously trip the protection. To re-enable operation the
VDD voltage has to be recycled.
Figure 36. OVP circuit
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Since the value of R3 has already been selected as a consequence of the maximum output
power that the converter has to deliver, R14 can be chosen according to the following
formula to reach the desired output overvoltage threshold, VOUT_OVP:
Equation 1
ROVP _(R14 ) =
RLIM _(R3 ) ⎛ NAUX
⎞
⋅ ⎜⎜
⋅ (VOUT _ OVP + Vγ D 4 − Vγ D 2 ) − VOVP ⎟⎟
VOVP ⎝ NSEC
⎠
where VγD2 and VγD4 are the forward drop of the diodes D2 and D4 respectively, NAUX and
NSEC are the number of turns of the auxiliary and secondary winding respectively. If the
overvoltage protection is not required, D2 and R14 can be removed.
The above formula, solved for VOUT_OVP, gives about 7.2 V for the setting of the presented
board.
This value has been verified experimentally, short-circuiting the lower resistor (R9) of the
output voltage divider and thus producing an output overvoltage, as shown in the figures
below. In Figure 37 one can see that as VOUT reaches the value of 7.2 V, the converter
stops switching. In the same figure, the CONT pin voltage and the FB pin voltage are
reported. The crest value of the CONT pin voltage during the MOSFET’s off time tracks the
output voltage. The converter is shut down when the CONT pin voltage reaches the 3 V
threshold, as shown in Figure 38.
Figure 37. Output overvoltage protection at
115 VIN_AC 0.2 A
Figure 38. Output voltage ripple at 230 VIN_AC
no load
!-V
!-V
The OVP protection is in auto-restart mode: after the shutdown for protection tripping, the
VDD recycles and, if the overvoltage is still present, the protection is tripped again,
indefinitely, until the cause of the fault is removed, at which point the converter resumes
normal operation, as shown in Figure 39.
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Figure 39. Auto-restart mode of the
overvoltage protection
Figure 40. Auto-restart mode of the
overvoltage protection (zoom)
!-V
!-V
The output OVP threshold has been measured in different line/load conditions and the
results are reported in Table 18.
Table 18.
Output overvoltage threshold at different input/output conditions
VIN [VAC]
VOUT_OVP [V]
IOUT [A]
90
115
230
265
0.01
7.15
7.16
7.26
7.3
0.5
7.27
7.26
7.27
7.31
1
7.22
7.25
7.34
7.4
2
7.3
7.32
7.4
7.41
In Figure 41, the same results – normalized with respect to the calculated VOUT_OVP
threshold (7.2 V) – are plotted in a graphic format to show that in any case the actual
threshold is within some percentage of tolerance of the expected value.
Figure 41. Normalized output OVP threshold vs load
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2.9
AN2950
EPT function
Some applications need a power higher than the nominal one to be supplied for a limited
time window, during which regulation of the converter has to be maintained.
In the VIPer28 this can be accomplished by the extra power management function, available
at the EPT pin. This function requires the use of a capacitor CEPT connected between the
pins EPT and GND (capacitor C12 in Figure 2).
During normal operation, the EPT pin voltage is zero. When, due to the output power
demand, the peak drain current rises over 85% of the IDlim value (IDlim_EPT, 0.68 A typical),
the CEPT capacitor is charged by an internal 5 µA current on a cycle-by-cycle basis.
If the extra power demand disappears before the EPT pin voltage has reached the
VEPT(STOP) threshold (4 V typical), the capacitor is discharged to zero at the same rate,
while the system continues working normally.
If the extra power duration is so long that the CEPT voltage reaches the VEPT(STOP)
threshold, the converter is shut down and the CEPT capacitor is discharged to zero by the
5 µA current.
Both cases are illustrated in Figure 42, for a load demand changing from
1 A to 3.3 A and back again to 1 A. After shutdown (Figure 43), the VDD voltage drops down
and has to fall below the VDD(RESTART) threshold (4.5 V typical) before the VDD capacitor
can be charged again up to VDDon. In any case, the PWM operation is only enabled again
after the EPT pin voltage has dropped below the VEPT(RESTART) threshold (0.6 V typical), as
illustrated in Figure 44.
The converter works indefinitely in this on-off condition until the extra power demand is
removed (Figure 42). This ensures safe operation and avoids overheating in case of
repeated overload events.
Figure 42. Extra power applied and removed
Figure 43. Extra power applied indefinitely
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Figure 44. EPT thresholds
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!-V
The time window TEPT during which the extra power is allowed to be supplied can be
chosen according to the following formula:
Equation 2
C EPT =
IEPT ⋅ TEPT
VEPT
The value of TEPT has to be chosen so as to prevent overheating of the VIPer28 and of the
power elements (usually up to a few seconds).
The EPT pin can be connected to GND if the function is not used.
2.10
Thermal measurements
A thermal analysis of the board has been performed using an IR camera. The results are
shown in Figure 45 for a 90 VAC mains input, full load condition.
Figure 45. Thermal measurements at 90 VAC full load, Tamb = 25 °C
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Table 19.
2.11
AN2950
Temperature of key components at 115 VAC full load
Point
Temperature (in oC)
Reference
A
66.6
D4 (output diode)
B
51.7
VIPer28LN
C
65.2
Dz (zener diode)
D
38.4
BR1 (Diode bridge)
E
38.9
D3 (clamp diode)
EMI measurements
A pre-compliant test to the EN55022 (class B) European norm has been performed using an
EMC analyzer and a LISN.
First of all, an average measurement of the background noise (board disconnected from the
mains) has been performed and is reported in Figure 46.
Figure 46. Average measurement of background noise (board disconnected from
mains)
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The average EMC measurements at 115 Vac/full load and 230 Vac/full load have then been
performed and the results are shown in Figure 47 and Figure 48 respectively.
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Figure 47. Average measurement at 115 Vac, full load
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Figure 48. Average measurements at 230 Vac, full load
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2.12
AN2950
Board layout
Figure 49. Top layer
!-V
Figure 50. Bottom layer
!-V
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3
Conclusions
Conclusions
The presented flyback converter is suitable for a wide range of applications. It can be used
as an external adapter or as an auxiliary power supply in consumer devices. Special
attention has been given to low-load performances and the bench results are good with very
low input power in light load conditions. The efficiency performances have been compared
with the requirements of the ENERGY STAR® program (version 2.0) for external AC/DC
adapters, with very good results showing that the measured active mode efficiency is always
higher than the minimum required.
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Test equipment and measurement of efficiency and low load performance
Appendix A
AN2950
Test equipment and measurement of
efficiency and low load performance
The converter’s input power has been measured using a wattmeter. The wattmeter
simultaneously measures the converter’s input current (using its internal ammeter) and
voltage (using its internal voltmeter). The wattmeter is a digital instrument, so it samples the
current and voltage and converts them into digital forms. The digital samples are then
multiplied, yielding the instantaneous measured power. The sampling frequency is in the
range of 20 kHz (or higher, depending on the instrument used). The display provides the
average measured power, averaging the instantaneous measured power in a short period of
time (1 sec typ.).
Figure 51 shows how the wattmeter is connected to the UUT (unit under test) and to the AC
source, as well as the wattmeter’s internal block diagram.
An electronic load has been connected to the output of the power converter (UUT), enabling
one to set and measure the converter’s load current, while the output voltage has been
measured by a voltmeter. The output power is the product of the load current vs. output
voltage.
The ratio between the output power, calculated as previously said, and the input power,
measured by the wattmeter, is the converter’s efficiency. It has been measured in different
input/output conditions acting on the AC source and on the electronic load.
A.1
Notes on input power measurement
This section shows two possible connections between the wattmeter and the unit under test
(UUT) for power measurements, each one represented in Figure 51 by the connection of the
switch either in position 1 or in position 2.
Figure 51. Connection of the UUT to the wattmeter for power measurements
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